"'Colloportus!' gasped Hermione and the door sealed itself with an odd squelching noise."

—Hermione Granger's use in the Battle of the Department of Mysteries[src]

The Locking Spell (Colloportus) is a charm that is used to lock or seal doors so that they cannot be opened manually; however, it is unknown whether a key could be used to unlock them.
If it is true that a key would be ineffective against a door sealed with this spell, it would prove quite useful against Muggles because there is a well-known counter charm, Alohomora.